by Colin Tudge
‘Diversity’, though (like all terms in biology), has various connotations. In this kind of context, it should not be measured purely in terms of number of species. We need to see how different the various species are, one from another – which is where molecular studies (of DNA) come into their own. Thus it may transpire that the twenty or so species of oak, or pine, or whatever in place A all have very similar DNA. Place B may have only half a dozen species, yet the difference in their DNA may be profound.
It would be reasonable to conclude that the species in place A all arose from a single ancestor, who arrived there fairly recently, found the place agreeable, and diverged rapidly (and perhaps re-hybridized, as outlined in Chapter 1). But the greater genetic diversity found in place B could be explained in two different ways. Perhaps all the trees did indeed arise in situ from a single founder, who arrived or originated in that place a great many years in the past, giving its descendants plenty of time to diverge. Or perhaps some at least of the very distinct trees originated in other places, and simply converged on the place that’s being studied. But then we can ask a further question. It is possible to infer from their DNA which species in any particular family (or which family in any particular order) is the most primitive – this being the one that seems to have most in common with the original ancestor. Common sense suggests, then, that sites that have the greatest true diversity of species (the greatest variation in DNA) and/or include the species that are known to be the most primitive, are at least reasonable candidates as the true centre of origin. But of course such a site could just be an ancient secondary centre of diversity. The trees might be completely extinct in the place where the group truly arose.
This is where fossils come to our aid. In more and more sites all around the world, palaeontologists are now finding fossils of truly astounding quality, that reveal the structure of ancient plants in the most minute detail. Pollen is particularly informative. It is highly characteristic, and often allows identification at the level of the genus. It is also very enduring, often to be found in the deepest mud beneath lakes, or in rock that derived from the mud of lakes that are now long gone. Pollen is to palaeobotanists what teeth are to scholars of ancient mammals. Fossil and sub-fossil pollen sometimes provides a continuous record of ancient floras over tens of millions of years.
Fossils can be deceptive, however. Fossilization is a rare event. The oldest fossils known of any particular group do not necessarily represent the very first of that group. Indeed – given that all groups are rare in their early stages – the oldest known fossils are most unlikely to represent the first that actually existed. Neither do the latest ones known necessarily represent the most recent. The most recent fossils of Metasequoia and Wollentia are both millions of years old; yet both these trees have proved to be alive and well, in China and Australia respectively.
But fossils do give us some certainties. If a fossil of a particular tree turns up in a rock that’s 100 million years old then we know that that tree did indeed live in that place, and that its species was there at least 100 million years ago. Thus we know for sure that the family Araucariaceae, now confined to the south, did once live in the northern hemisphere. Contrariwise, the absence of southern beech fossils in the northern hemisphere does not prove that the southern beeches never came north of the equator. As the adage has it, ‘Absence of evidence is not evidence of absence.’ Even so, the fact that thousands of trawls from hundreds of sites over many decades have failed to produce any southern beech pollen in the north is at least a strong suggestion that they have always been out-and-out southerners. Southern beeches, presumably, really did arise in the southern hemisphere.
But there is another huge complication. Conifers as a whole first arose several hundred million years ago, and some modern conifer families are well over 100 million years old. Flowering plants as a group are much younger, but still many families of them are tens of millions of years old. Since the time when many plant families began, much of the land on which they stand has moved dramatically.
AND YET THEY MOVED
The first suggestion that the continents are moving around the globe came from the German geologist Alfred Wegener, who in 1915 coined the expression that translates as ‘continental drift’. Wegener found that if you cut up a map of the world and shuffle the bits around then the existing continents and the big islands, particularly of Australia, Africa, South America and Antarctica (and Madagascar and New Zealand), fit together like a toddler’s jigsaw. In the north, the eastern coasts of North America, plus Greenland and Iceland, when shoved sideways, abut neatly with the west coast of Europe. The Atlantic is shaped like a snake. The coincidences just seemed too great. Surely, he said, the different continents and islands must once have been joined together, then split and drifted apart. At first, many scientists were thrilled with Wegener’s idea. Then they decided that it was impossible (meaning that they could not think of a mechanism) and most then declared that it was obviously ludicrous. By the time of his death, in 1930, he was more or less outcast. Only a few brave hearts supported him.
But the brave hearts turned out to be right. The continents have moved, and, measurably, are still moving. The mechanism that drives them began to become apparent after the 1950s. The centre of the earth is hot, as indeed had been known for some decades. Indeed it is so hot that the entire interior swirls with convection currents, like a simmering kettle. The interior rock is the magma that flows out when volcanoes erupt. The continents are made of lighter rock, and float on the restless magma like froth on a slow-moving river.
The continents move slowly – only a few centimetres a year – but they have had plenty of time, and over the past few billions of years their peregrinations have been prodigious. Five hundred million years ago (in Cambrian times) the present land masses were scattered islands. Places now in the tropics have at times been near the poles, and vice versa. Places that are now in the heart of continents have been islands, and present-day islands have been part of mighty continents. Most of what is now North America was an island that straddled the equator. Siberia was a subtropical island in the southern hemisphere. And so on.
All this was before any plants had come on to land, and long before there were trees. But by about 265 million years ago (the mid-Permian), when already there were plenty of trees, all the islands had massed together to form one great land mass known as Pangaea. By about 200 million years ago (in the early Jurassic), when the conifers and cycads were in their pomp and the flowering plants were yet to come on the scene, Pangaea began to split more or less in half to form two great ‘supercontinents’: Laurasia to the north, and Gondwana to the south.
About 225 million years ago the world’s continents were joined to form a supercontinent called Pangaea.
Eventually Pangaea started to break into two separate continents.
Gondwana broke up to form Antarctica, Africa, Australia and South America; plus India, Madagascar, New Zealand and New Caledonia. Many (though by no means all) of the trees that now live in those Southern lands were Gondwanan in origin
The continents are still drifting. Perhaps in a few million years Australia will collide with Southern Asia, as India once did. Perhaps it will crunch into Japan. Or perhaps it will slide past Japan and into the North Pacific. Each of the possible scenarios will be dramatic, though none is urgent.
Ever since – through all the time that the flowering plants have been evolving – those two great supercontinents have been breaking up. Laurasia has split to form present-day North America, Greenland, Europe and most of Asia. Gondwana has fragmented to become present-day Antarctica (a huge continent), South America, Africa, Arabia, Madagascar, India and Australia, plus a fairly long catalogue of today’s islands including New Zealand and New Caledonia.
The details of this redistribution are roughly outlined in the figure on pp. 284–5. Two features in particular are outstanding. Note, first, that South America was an island for tens of millions of years, until it
finally made contact with North America, via what is now called Panama, around 3 million years ago – which in geological time is very recent. India too was an island that drifted north for tens of millions of years until it finally crunched into the south of Asia. The crunch took place about 60 million years ago (not long after the dinosaurs disappeared) – and the impact, slow and inexorable as it was, caused the rise of the Himalayas (and the Himalayas would still be rising were it not for the erosion that tends to keep them at the same height). Australia is still an island, drifting north. Opinion is divided on whether it will miss Asia altogether, or crunch into China, or obliterate Japan.
All the time the continents have been shifting, so the modern groups of plants and animals have been emerging. Modern land mammals, which could not easily cross the widening oceans, still show its effects very clearly. So it was that in the nineteenth century Alfred Russel Wallace pointed out that the mammals of South-East Asia were quite distinct from those of Australia and New Guinea. In Asia there were and are cats, pigs, deer; while Australia was and is dominated by marsupials, and contained the unique egg-laying monotremes – the platypus and echidnas (though there is also an echidna in New Guinea). The boundary between the two faunas became known as ‘the Wallace Line’. Although Wallace didn’t know it, his line marks the border of Laurasia and Gondwana.
Continental drift must also enrich our attempts to explain why modern trees are where they are. To be sure, each kind must have arisen in one particular place, its centre of origin. Each may then have spread to other places, and then spread again, and perhaps the earliest ones in the original centre went extinct. But all the time this was happening the ground itself was shifting, as the continents wandered the globe, breaking apart and sometimes joining up with others.
Through such ideas we begin to see why different modern-day islands have such very different characters. Madagascar has been an island for a very long time but it used to be part of Gondwana. When it broke from the mother continent it contained a random collection of plants and animals – the latter (perhaps) including the ancestors of lemurs, but not of modern carnivores or hoofed animals. Whatever plants it had on board were free to evolve in all kinds of strange directions, without competition from continental types. The traveller’s palm, Ravenala, the baobabs, and the Didiereaceae are among the results. New Caledonia too is a fair-sized, long-isolated fragment of the erstwhile Gondwana; but New Caledonia began its island existence with a different collection of Gondwanan creatures on board which now are almost as spectacularly strange as those of Madagascar. In contrast, Hawaii, the Galapagos, the Azores and the Canaries did not begin as bits of continents. They arose from the bottom of the ocean, in volcanic eruptions, and began life as bare rock. They too have their own unique collections of endemic creatures. But all their inhabitants are descended from ancestors that were brought in by wind or water, or flew or swam, or hitched a ride on the flyers or swimmers. Creatures with no such abilities stayed away. Conifers (apart from one bird-dispersed juniper on the Azores) are notably absent from mid-oceanic volcanic islands. New Caledonia is in the middle of nowhere but it is a fragment of continent, and its indigenous conifers, borne from Gondwana, are among the wonders of the world.
Britain was also part of an ancient continent, as Madagascar was, and has been linked to mainland Eurasia from time to time in the past few million years as the sea level fell during the ice ages. Yet it lacks much of the variety found in neighbouring Eurasia and has no endemics to speak of, apart from a few fish, including various subspecies of char and trout. But here we encounter yet another complication. Britain endured the rigours of the ice ages. During the ice ages the British Isles were linked to mainland Europe, which should have enriched its flora and fauna. But instead, or at least equally, the ice wiped out what was there. Madagascar has long been in tropical latitudes, and escaped any such purge. This story belongs to the next discussion, however: why the tropics are so much more various than temperate latitudes.
With these general principles in place (centre of origin, secondary centres of diversity, continental drift); and aided by modern techniques (analysis of DNA and the steadily improving fossil record), biologists are now putting flesh on the theoretical bones. The Royal Society meeting of March 2004 showed how rich – and unexpected – the realities are now proving to be.
REALITY: A FEW CASE HISTORIES
Take, for example, the American tropics (the neotropics), the most diverse ecosystem of all. South America is a fragment of Gondwana that broke away from Antarctica some time in the Cretaceous, rafted north as an island for the best part of 100 million years, and finally docked with North America around 3 million years ago. (Although ‘finally’ in this context simply means ‘most recently’. The travels of continents, like the Flying Dutchman’s, will never end.) The relationships of animals can be easier to see than those of plants, and it’s clear that South America has many strange animals – notably the sloths, anteaters, armadillos and its own suite of marsupials – that are peculiar to itself, and reflect the Gondwanan origins of South America, and its long isolation as an island. But South America also contains northerners, including jaguars, pumas and various deer, which came in from North America. So we might expect to see the same pattern among trees: basically a Gondwanan island flora, with a few incursions, mainly from North America.
Yet this commonsense assumption holds up only to a very limited extent. Thus, the characteristic flora of northern South America at present is tropical rainforest. If the trees of the forest are indeed Gondwanan in origin then their ancestors ought to have been there tens of millions of years ago. But Dr Robyn Burnham of the University of Michigan has studied fossil plants in Bolivia and found very little evidence of any tropical rainforest at all before about 60 million years ago. The oldest rainforest fossils seem to date from around 50 million years ago. But she has found clear evidence of tropical rainforest in several sites in North America that are more than 60 million years old. This suggests (it does not prove, but it suggests) that the great rainforest of the neotropics, including South America, arose in the northern continent, Laurasia; and that the trees found their way to South America long before South America met up with North America around 3 million years ago, in the Pliocene. This fits in with other evidence of many kinds which suggests that South America had contact of a kind with northern continents long before the Pliocene. For example, the small south American cats – the ocelot and margay – seem to have been in place since well before the Pliocene. So too have South America’s monkeys, like the howlers, capucins and tamarins. The ancestors of these animals also arose on other continents but they too got to South America before North and South America most recently collided. Presumably there were other land bridges in the past, now long gone – or if not literal bridges, then at least chains of islands, as are still to be seen through the Caribbean.
However, molecular evidence from Dr Toby Pennington, at the Royal Botanic Gardens, Edinburgh, strongly suggests that the present-day trees of the South and Central American forest did not come from North America either. Their closest relatives, at least of the groups he has looked at, often seem to be in Africa. This seems to take us back to the original idea – that the plants of Africa and South America are both simply Gondwanan. But close comparison of DNA suggests that many of the present-day South American plants originated after South America broke from Gondwana – and that by some means or other they have entered from Africa. The picture grows more and more mysterious.
As with South America, so with Australia – but with a different twist. As with South America, we would expect from first principles that its flora, including its trees, would basically be a selection of those that were originally on Gondwana. Again, up to a point, this is true. But again there are some serious complications.
Thus it is that the present-day vegetation of southern Australia is very varied. Some is alpine (low mountain). Much is shrubland and grassland. Grasses came only very recently to A
ustralia, but much of the rest, as we would expect, is basically Gondwanan.
But Robert Hill, at the University of Adelaide, concludes from the fossils that when Australia first parted company with the rest of Gondwana around 70 million years ago, the south of Australia was largely covered in rainforest. (Strange to think of rainforest inherited from Antarctica; but then the world is strange.) But as Australia moved north it was cooled by the cold current that began to circulate around Antartica. As the island continent cooled so it dried, and by 5 million years ago much of it was arid. Australia, too, is heavily eroded and short of nutrients – so the plants had to cope both with lack of nutrient and with lack of water. In addition, Australia has been beset by ice ages.
Specifically, Mike Crisp of the Australian National University in Canberra concludes that Australia inherited southern beeches from Gondwana, and that these sat around for a bit and then diversified impressively. But then the aridity got to them, and now there are very few. So the southern beeches were largely replaced by the eucalypts, which cope with aridity very well. Professor Hill feels that the eucalypts were a late development, and indeed may have arisen within Australia itself. In contrast, Professor Crisp believes that, like southern beech and indeed like the Banksias (in the same family as Grevillea, the Proteaceae), the eucalypts arose in Gondwana and were present in Australia from the start of its career as an island. Where Australia’s acacias (in those parts known as wattles) came from is not obvious, but they clearly diversified mightily as the continent became more arid.
In general, too, the picture of Australia’s trees and other plants has been hugely complicated by various animals, including humans. Australia used to have some bigger mammals than it has now, including an (almost) rhino-sized wombat called Diprotodon and a giant kangaroo that stood around 3 metres tall. There were more huge reptiles, too – which in Australia’s early days as a solo land mass included dinosaurs. Some of these may well have helped to disperse the seeds of Australia’s early trees; and with them gone, the trees would suffer. The large mammals (including Diprotodon) evidently became extinct some time after the Aborigines arrived from South-East Asia, at least 40,000 years ago and perhaps as long as 80,000 years ago. Aborigines, too, have long made extensive use of fire to encourage grasses and so attract animals for hunting. Their fires may well have altered the vegetation of central Australia irrevocably. The Europeans, of course, who probably first set foot on Australia in the seventeenth century and finally got to grips with it through James Cook’s voyages in the eighteenth, have transformed much of the country, with a huge array of imported plants and animals including quasi-wild creatures such as rabbits and foxes (which affect the local marsupials, which in turn interact with the plants) and domestics such as sheep, water buffalo and cats. Even in the deep past, however — millions of years before human beings arrived, and indeed before they arose as a species – the pristine Gondwanan flora that Australia inherited was complicated and largely transformed by climate, erosion, and the native animals.
